1. Field of the Invention
The present invention relates to a vacuum pump.
2. Description of the Related Art
A vacuum pump represented by a turbo-molecular pump is attached to a vacuum chamber of a dry etching apparatus, a CVD apparatus, etc. The turbo-molecular pump includes a rotor having rotor blades and a rotor cylinder, stator blades opposed to the rotor blades, and a screw stator radially opposed to the rotor cylinder. The rotor rotates at a high speed of tens of thousands of revolutions per minute. The revolution of the rotor causes the rotor blades and the stator blades to cooperate and the rotor cylinder and the screw stator to cooperate to discharge gas in the vacuum chamber and create a high vacuum in the vacuum chamber.
Most of the gas discharged by the rotor cylinder and the screw stator is directed to an exhaust port, while part of the gas may be directed to an inner peripheral side of the rotor. The gas discharged from the vacuum chamber contains corrosive process gas, and this process gas enters a housing (hereinafter referred to as a “motor housing”) provided on the inner peripheral side of the rotor and corrodes electrical systems such as a magnetic bearing device and a motor. In order to prevent this problem, an outer peripheral surface of the motor housing can be provided with a thread groove pumping element.
However, various reaction products, which are included in the gas discharged by the rotor cylinder and the screw stator, may be deposited in the thread groove pumping element provided on the outer peripheral surface of the above motor housing. Since the rotor is expanded by centrifugal force during pump operation, a gap between the rotor cylinder and the motor housing is larger during the pump operation than when stopped. Thus, when reaction products are deposited during the pump operation and then a pump is stopped and the expansion of the rotor is restored, an inner peripheral surface of the rotor cylinder and the outer peripheral surface of the motor housing may adhere by the reaction products.
JP H05-006195 Y discloses a device that prevents gas discharged by a rotor cylinder and a screw stator from being directed to an inner peripheral side of a rotor by thread grooves formed integrally with a bottom of a base in a position opposite to the rotor cylinder on the bottom of the base.
The device disclosed in JP H05-006195 Y, however, is provided with a thread groove pumping element in an exhaust path and this may results in degradation of exhaust performance.